1. Field of the Invention
The present invention relates to a speaker having a diaphragm, a coil and a magnetic circuit, and more particularly, to a speaker which provides magnetic damping when amplitudes of the diaphragm and coil become large.
2. Description of the Related Art
FIG. 5 is a sectional view showing a conventional dynamic-type speaker typically used, for example, as a vehicle-mounted speaker; FIG. 6A is a sectional view showing a magnetic circuit of the conventional speaker; and FIG. 6B is a diagram showing the magnetic flux density produced in the magnetic circuit.
The conventional speaker shown in FIG. 5 includes a frame 1 made of metal, and a diaphragm 2 supported within the frame 1. The diaphragm 2 is cone-shaped and made of a paper material. A semispherical dome portion 3 is attached over an opening formed in a central portion 2a of the diaphragm 2. A suspension 4 is attached between the frame 1 and an outer portion 2b of the diaphragm 2. The suspension 4 is provided separately from the diaphragm 2 along the circumference of the edge of a outer portion 2b of the diaphragm 2. The suspension 4 includes a substantially semi-circular (semicylindrical) cross sectional configuration with a predetermined curvature. An inner edge of the suspension 4 is bonded to the diaphragm 2 and an outer edge is bonded to an outer edge 1a of the frame 1.
The outer periphery of the central portion 2a of the diaphragm 2 is supported by the frame 1 through a damper 5. The damper 5 is corrugated to form multiple ridges which are concentric about a central axis (Y-axis) of the speaker. An inner edge of the damper 5 is bonded to the central portion 2a of the diaphragm 2 and an outer edge thereof is bonded to the inner surface of the frame 1. The diaphragm 2 is supported by the suspension 4 and the damper 5 such that it can vibrate in the Y-axis direction.
A magnetic circuit A is provided between a bottom 1a of the frame 1 and the central portion 2a of the diaphragm 2. The magnetic circuit A is composed of a cylindrical yoke 6 made of a magnetic material, a ring-shaped magnet 7 positioned at the outer periphery of the yoke 6 and ring-shaped magnetic pole pieces 8a and 8b which joined to upper and lower ends of the magnet 7. The magnet 7 has magnetic anisotropy in the Y-axis direction such that a north (N) pole is formed near the upper end of the magnet 7 and a south (S) pole is formed near the lower end of the magnet 7. The yoke 6 and ring-shaped magnetic pole pieces 8a and 8b are formed of a material having a high magnetic permeability such as soft iron. Gaps G1 and G2 are respectively formed between the inner peripheral surfaces of the ring-shaped magnetic pole pieces 8a and 8b and the outer peripheral surfaces of the yoke 6.
A bobbin 9 is joined to the central portion 2a of the diaphragm 2 and extends into the gaps G1 and G2. Coils Cl and C2 are mounted on the bobbin 9 and are spaced apart in the Y-axis direction such that the coil Cl is positioned within the gap GI and the coil C2 is positioned within the gap G2.
The coils Cl and C2 are connected in series and wound such that they generate electromagnetic forces in the same direction when a predetermined voice current is applied thereto.
FIG. 6A shows an enlarged magnetic circuit A of the conventional speaker. As described above, in the conventional magnetic circuit A, the gaps G1 and G2 are respectively formed where ring-shaped pole pieces 8a and 8b face the yoke 6. Thus, within the widths W of the gaps G1 and G2, the magnetic flux density between the yoke 6 and the ring-shaped pole pieces 8a and 8b is high, and the magnetic flux density decreases substantially outside the widths W of the gaps G1 and G2, even when the distance outside the widths W is small.
FIG. 6B shows a change in the magnetic flux density in the Y-axis direction. It is apparent from this drawing that the magnetic flux density within the range of the widths W greatly differs from that in the range outside the widths W. When the coils C1 and C2 each having the width B in the Y-axis direction fall within the widths W of the gaps G1 and G2, the magnetic density crossing the coils C1 and C2 is substantially uniform, and a linear electromagnetic force acts on the coils.
However, as shown in FIG. 6B, substantially both ends of the widths W of the gaps G1 and G2 are sudden change points "a" where the magnetic flux density falls off suddenly. Therefore, as shown in FIG. 6A, when vibration of the diaphragm 2 causes the coils C1 and C2 to be located outside of the widths W of the gaps G1 and G2, a linearity of the electromagnetic force acting on the coils C1 and C2 is extremely deteriorated by an influence of the sudden change points "a", thereby causing sound distortion.
A vehicle-mounted speaker and the like is conventionally used for amplifying a radio sound and reproduced sound stored on a magnetic tape. Recently, however, these speakers have been used for reproducing music signals from sources such as compact discs (CDs).
In reproducing such music signals, an amplifying peak of the sound is often increased due to extension of a dynamic range, thereby repeatedly applying excessive input signals to the speaker. These excessive input signals cause the coils C1 and C2 to move outside the widths W of the gaps G1, G2 such that sound distortion frequently occurs.
In addition, when the vibration amplitudes of the diaphragm 2 and bobbin 9 in the Y-axis direction become large due to the excessive input signals, the damper 15 impacts the upper surface of the ring-shaped pole piece 8a, or the lower end 9a of the bobbin 9 impacts the inner wall of a bottom lb of the frame 1, thereby causing frequent impact sounds. When an extremely large input is provided, the damper 5 and bobbin 9 may be damaged by these impacts.
In addition, when the vibration amplitude of the diaphragm 2 in the Y-axis direction becomes large, the amplitude cannot be absorbed by the suspension 4, and the semicylindrical portion of the suspension 4 is excessively stretched or contracted such that sound distortion is caused. Further, when extremely large input signals are intermittently provided, the suspension 4 may be damaged.